On the colour reactions of potato glycoalkaloids in strong acids in the presence of paraformaldehyde

The colour reaction of potato glycoalkaloids, containing solanidine as steroid, with strong acids and paraformaldehyde was examined to elucidate the kinetics and mutual relationship of the ‘Clarke’ and ‘Marquis’ reactions. It was found that the influence of time of addition of paraformaldehyde was the major factor in deciding the type of colour changes encountered. This factor and the specific absorption kinetics have important consequences for the use of these types of colour reactions as a quantitative assay. According to the structural similarity of the steroidal part of potato glycoalkaloids and cholesterol, colour reactions of both have been compared, indicating that both steroids react in strong acids in the presence of an oxidator, provided that the more lipophilic character of cholesterol is taken into account. Therefore a similar mechanism of a serial oxidation of carbonium ions as in the case of cholesterol, is suggested as the basis for these specific colour reactions of potato glycoalkaloids.